Harman Kardon PM650 Indication Lights - UK Vintage Radio Repair and Restoration Discussion Forum

Bookman · 22nd Jan 2019, 3:46 pm

I could not resist buying one of the above from a well known web site and was pleased at what I received for the money paid.
Apart from a few glaring issues that are not particularly difficult to sort I could do with a bit of guidance with the indication lights; pics attached.
As can be seen, they are of an insert and twist configuration. A bit of corrosion has occurred but it did not prevent their removal; albeit one was missing.
My question is does anyone know if such items can still be sourced and if so, from whom?
They are incandescent operative from a 20V DC source.
The latter obviously permits the optional use of LEDs without any radical mods and here again would anyone have any suggestions?
Any help would be appreciated
Thanks
Joe

paulsherwin · 22nd Jan 2019, 4:53 pm

Search for 'grain of wheat' bulbs, though you may have trouble finding 20V ones. 24V ones will probably be OK.

https://www.ebay.co.uk/itm/50-pcs-5m...s/400109549060

If you are certain the supply is DC, you can easily substitute a white LED. You can experiment with whatever you have to hand, adjusting the series resistor to get the right brightness. It will work with AC too, but that might result in visible flicker.

If you don't want focussed light from the LED, you can crack off the plastic lens with a pair of sidecutters.

Techman · 22nd Jan 2019, 5:54 pm

Quote:

Originally Posted by Bookman

one was missing. They are incandescent operative from a 20V DC source.

If they were all out, it's worth checking that they're not lower voltage types connected in series like Christmas tree lights. I've found that Grundig sometimes like to do this sort of thing!

Bookman · 22nd Jan 2019, 7:40 pm

Thanks to Paul and Techman.
I was actually thinking of xmas tree lights as a possible option indeed I am a switchgear guy and had not heard of "grain of wheat nomenclature" previously so yet another variable for consideration?
Incidentally, my reference to 20V DC is in fact the measured Voltage taken on the PC Board and that each bulb has its own circuitry. Yes, it is an open circuit value but with only 6 bulbs in circuit I doubt the percentage regulation will reduce the value much. Furthermore, the PM 650 has a plastic cover that has green and red tape and as such there is no white colour. Ergo, a dim 24V rated item ought to suffice.
Cant wait to test the item as I am eager to see what effect the "Cartridge Trim Capacitor selector switch" has.

snowman_al · 22nd Jan 2019, 7:56 pm

The service manual is freely available on the web. Try here https://www.vintageshifi.com/reperto...H-Kardon-4.php

Circuit page 7 and Lamp panel page 14.

Its shows them as 36mA jobbies. I think the 110 volt bit is a mis-print and 20 / 24 volts would be fine.
As would using LEDs and series resistors in their place.

Alan

paulsherwin · 22nd Jan 2019, 8:33 pm

Be careful with Christmas tree bulbs. Most of them are designed to fail short circuit so that the other lamps in the string stay lit. They will eventually burn out if subjected to sufficient current, but it's probably not what you want. The 'fuse bulbs' with the white paint on the end do fail open circuit, but there aren't so many of those around.

mhennessy · 22nd Jan 2019, 9:05 pm

Of course, you need enough voltage to jump the open circuit and fuse it short. For the Noma lights I use, it's about 100 to 150V - I know this because I run my tree lights from a variac

Looking at the service manual from Hi-Fi Engine, they run from 12V DC (barely smoothed), because all but one of them is switched by a transistor. So that makes an LED mod slightly easier. I'd increase the value of C12, as otherwise they will flicker at 100Hz - though not all would notice or be bothered by that.

paulsherwin · 22nd Jan 2019, 9:36 pm

In that case, they could actually be 12V bulbs, which will be much easier to find.

I think an LED conversion would be easier and cheaper overall though - easy enough to try with one of them anyway.

Bookman · 23rd Jan 2019, 10:34 am

Many thanks to everyone that has responded. Luckily component costs in this situation are such that they will allow a trial and error exercise without due worries. Perhaps more importantly is the ease of access provided.
Regards
Joe

Lloyd 1985 · 23rd Jan 2019, 10:58 am

I’m sure I’ve seen those bulb types before, in a car clock that came from a Rover, one where the display was separate from the radio. I just remember the funny little plastic twist holder for the bulb.

Regards
Lloyd

Edit: just found this! You can get drop in LED replacements on the usual auction site, and googling around showed up some proper incandescent bulb versions too.

Herald1360 · 23rd Jan 2019, 11:13 am

I thought "dashboard lamp" but the low wattage wouldn't fit. I'd go with the LED upgrade. Modern 1/4W resistors are smaller than a 3mm LED, let alone a 5mm one so all very easy to do. It doesn't matter which leg the resistor goes in, but it's worth noting which way round the lights will now have to be fitted. The PCB layout may mean they're not all the same.

turretslug · 23rd Jan 2019, 1:35 pm

Will the replacement LED types that Lloyd linked to actually need the resistors, or even attention to polarity? I assume that they are made as drop-in replacements for "appliance operators", rather than needing any application of technical awareness, with an integrated bridge and current limiting built-in.

Obviously, a point to check with a sample device and something like a bench PSU!,

Colin

22nd Jan 2019, 3:46 pm	#1
Bookman Hexode Join Date: Aug 2017 Location: Taunton, Somerset, UK. Posts: 318	Harman Kardon PM650 Indication Lights I could not resist buying one of the above from a well known web site and was pleased at what I received for the money paid. Apart from a few glaring issues that are not particularly difficult to sort I could do with a bit of guidance with the indication lights; pics attached. As can be seen, they are of an insert and twist configuration. A bit of corrosion has occurred but it did not prevent their removal; albeit one was missing. My question is does anyone know if such items can still be sourced and if so, from whom? They are incandescent operative from a 20V DC source. The latter obviously permits the optional use of LEDs without any radical mods and here again would anyone have any suggestions? Any help would be appreciated Thanks Joe Attached Thumbnails

22nd Jan 2019, 4:53 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,940	Re: Harman Kardon PM650 Indication Lights Search for 'grain of wheat' bulbs, though you may have trouble finding 20V ones. 24V ones will probably be OK. https://www.ebay.co.uk/itm/50-pcs-5m...s/400109549060 If you are certain the supply is DC, you can easily substitute a white LED. You can experiment with whatever you have to hand, adjusting the series resistor to get the right brightness. It will work with AC too, but that might result in visible flicker. If you don't want focussed light from the LED, you can crack off the plastic lens with a pair of sidecutters.

22nd Jan 2019, 7:40 pm	#4
Bookman Hexode Join Date: Aug 2017 Location: Taunton, Somerset, UK. Posts: 318	Re: Harman Kardon PM650 Indication Lights Thanks to Paul and Techman. I was actually thinking of xmas tree lights as a possible option indeed I am a switchgear guy and had not heard of "grain of wheat nomenclature" previously so yet another variable for consideration? Incidentally, my reference to 20V DC is in fact the measured Voltage taken on the PC Board and that each bulb has its own circuitry. Yes, it is an open circuit value but with only 6 bulbs in circuit I doubt the percentage regulation will reduce the value much. Furthermore, the PM 650 has a plastic cover that has green and red tape and as such there is no white colour. Ergo, a dim 24V rated item ought to suffice. Cant wait to test the item as I am eager to see what effect the "Cartridge Trim Capacitor selector switch" has.

22nd Jan 2019, 7:56 pm	#5
snowman_al Octode Join Date: Dec 2018 Location: Llandeilo, West Wales, UK. Posts: 1,092	Re: Harman Kardon PM650 Indication Lights The service manual is freely available on the web. Try here https://www.vintageshifi.com/reperto...H-Kardon-4.php Circuit page 7 and Lamp panel page 14. Its shows them as 36mA jobbies. I think the 110 volt bit is a mis-print and 20 / 24 volts would be fine. As would using LEDs and series resistors in their place. Alan Last edited by snowman_al; 22nd Jan 2019 at 8:14 pm. Reason: typo

22nd Jan 2019, 8:33 pm	#6
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,940	Re: Harman Kardon PM650 Indication Lights Be careful with Christmas tree bulbs. Most of them are designed to fail short circuit so that the other lamps in the string stay lit. They will eventually burn out if subjected to sufficient current, but it's probably not what you want. The 'fuse bulbs' with the white paint on the end do fail open circuit, but there aren't so many of those around.

22nd Jan 2019, 9:05 pm	#7
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Harman Kardon PM650 Indication Lights Of course, you need enough voltage to jump the open circuit and fuse it short. For the Noma lights I use, it's about 100 to 150V - I know this because I run my tree lights from a variac Looking at the service manual from Hi-Fi Engine, they run from 12V DC (barely smoothed), because all but one of them is switched by a transistor. So that makes an LED mod slightly easier. I'd increase the value of C12, as otherwise they will flicker at 100Hz - though not all would notice or be bothered by that.

22nd Jan 2019, 9:36 pm	#8
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,940	Re: Harman Kardon PM650 Indication Lights In that case, they could actually be 12V bulbs, which will be much easier to find. I think an LED conversion would be easier and cheaper overall though - easy enough to try with one of them anyway.

23rd Jan 2019, 10:34 am	#9
Bookman Hexode Join Date: Aug 2017 Location: Taunton, Somerset, UK. Posts: 318	Re: Harman Kardon PM650 Indication Lights Many thanks to everyone that has responded. Luckily component costs in this situation are such that they will allow a trial and error exercise without due worries. Perhaps more importantly is the ease of access provided. Regards Joe

23rd Jan 2019, 10:58 am	#10
Lloyd 1985 Nonode Join Date: Jun 2008 Location: Coningsby, Lincolnshire, UK. Posts: 2,819	Re: Harman Kardon PM650 Indication Lights I’m sure I’ve seen those bulb types before, in a car clock that came from a Rover, one where the display was separate from the radio. I just remember the funny little plastic twist holder for the bulb. Regards Lloyd Edit: just found this! You can get drop in LED replacements on the usual auction site, and googling around showed up some proper incandescent bulb versions too. Attached Thumbnails Last edited by Lloyd 1985; 23rd Jan 2019 at 11:08 am.

23rd Jan 2019, 11:13 am	#11
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Harman Kardon PM650 Indication Lights I thought "dashboard lamp" but the low wattage wouldn't fit. I'd go with the LED upgrade. Modern 1/4W resistors are smaller than a 3mm LED, let alone a 5mm one so all very easy to do. It doesn't matter which leg the resistor goes in, but it's worth noting which way round the lights will now have to be fitted. The PCB layout may mean they're not all the same. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

23rd Jan 2019, 1:35 pm	#12
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,394	Re: Harman Kardon PM650 Indication Lights Will the replacement LED types that Lloyd linked to actually need the resistors, or even attention to polarity? I assume that they are made as drop-in replacements for "appliance operators", rather than needing any application of technical awareness, with an integrated bridge and current limiting built-in. Obviously, a point to check with a sample device and something like a bench PSU!, Colin